Optical stack for privacy display

ABSTRACT

A switchable privacy display apparatus comprises a spatial light modulator, a light control film, and a polar control retarder that comprises plural retarders arranged between a display polariser of the spatial light modulator and an additional polariser. The display achieves high image visibility to an off-axis user in a public mode of operation and high image security to an off-axis snooper in privacy mode of operation.

TECHNICAL FIELD

This disclosure generally relates to optical stacks for use in privacydisplay and low stray light displays.

BACKGROUND

Privacy displays provide image visibility to a primary user that istypically in an on-axis position and reduced visibility of image contentto a snooper, that is typically in an off-axis position.

Switchable privacy displays may be provided by control of the off-axisoptical output.

Control of off-axis privacy may be provided by means of contrastreduction, for example by adjusting the liquid crystal bias tilt in anIn-Plane-Switching LCD.

Control may be further provided by means of off-axis luminancereduction. Luminance reduction may be achieved by means of switchablebacklights for a liquid crystal display (LCD) spatial light modulator.Off-axis luminance reduction may also be provided by switchable liquidcrystal retarders and compensation retarders arranged to modulate theinput and/or output directional luminance profile of a spatial lightmodulator.

Control may be further provided by means of off-axis reflectivityincrease. Reflectivity increase may be achieved by means of switchableliquid crystal retarders, compensation retarders that are arranged tocontrol the polarisation of ambient light that falls onto a reflectivepolariser.

BRIEF SUMMARY

According to a first aspect of the present disclosure there is provideda display device comprising: a spatial light modulator arranged tooutput spatially modulated light, the spatial light modulator includinga display polariser arranged on a side of the spatial light modulator,the display polariser being a linear polariser; an additional polariserarranged on the same side of the spatial light modulator as the displaypolariser, the additional polariser being a linear polariser; at leastone polar control retarder arranged between the additional polariser andthe display polariser, and a light control film arranged in series withthe spatial light modulator, the additional polariser and the at leastone polar control retarder, wherein the light control film comprises aninput surface, an output surface facing the input surface, an array oftransmissive regions extending between the input surface and the outputsurface, and absorptive regions between the transmissive regions andextend at least partway between the input surface and the outputsurface. Advantageously a switchable privacy display may be providedwith regions of increased security factor and with increased size ofpolar regions over which desirable security factor is achieved. In apublic mode of operation, off-axis users may be provided with increasedimage visibility.

The light control film may have a transmittance that is 5% or more in arange of polar angles in a direction in which the array of transmissiveregions repeats that may be at least 80° wide, be at least 90° wide andmay be at most 130° wide. The absorptive regions of the light controlfilm may have a thickness, t, wherein t may be given by the expression:

t=(S _(IN) −S _(IN) ²/10p+S _(OUT))/(2*tan(a sin(ξ/n)))

where S_(IN) is a width of an aperture of the input end of theabsorptive regions, S_(OUT) is a width of an aperture of the output endof the absorptive regions, p is a pitch of the transmissive regions inthe direction in which the array of transmissive regions repeats, and nis the refractive index of the transmissive regions; wherein ξ may be0.643 or more, ξ may be 0.707 or more and ξ may be 0.906 or less.Advantageously increased image visibility may be achieved in a publicmode of operation for off-axis users.

The light control film may have a transmittance that may have profileswith polar angle in a direction in which the array of transmissiveregions repeats that have centre lines directed inwardly towards anoptical axis extending forwardly from the centre of the spatial lightmodulator. Said centre lines of said profiles may be directed towards acommon point. The light control film may have a transmittance that mayhave a profile with polar angle in a direction in which the array oftransmissive regions repeats that is centred on the normal to the planeof the spatial light modulator at all positions across the light controlfilm. The transmissive regions may be tilted so that axes defined inrespect of each transmissive region between centres of apertures ofinput and output ends of the transmissive regions may be directedinwardly towards an optical axis extending forwardly from the centre ofthe spatial light modulator. Said axes may be directed towards a commonpoint. Advantageously image uniformity of luminance may be increased fora primary display user. Uniformity of image security may be increasedfor off-axis snoopers.

The transmissive regions have axes defined in respect of eachtransmissive region between centres of apertures of input and outputends of the transmissive regions may be normal to the plane of thespatial light modulator at all positions across the light control film.Advantageously the light control film may be conveniently tooled at lowcost.

The array of transmissive regions may be a one-dimensional array ofelongate transmissive regions. Advantageously increased transmission isachieved for desirable image security levels.

The absorptive regions between the transmissive regions extend betweenthe input surface and the output surface. The light control film may beprovided on a support substrate. High uniformity of alignment of thelight transmission regions may be obtained and display uniformity may beincreased.

Said display polariser may be an output display polariser arranged onthe output side of the spatial light modulator. The display devicefurther comprises a reflective polariser arranged between the outputdisplay polariser and at least one first polar control retarder, thereflective polariser being a linear polariser. A privacy mode may beprovided with increased reflectivity for off-axis snooper locations.Advantageously image security factor may be increased in environmentswith ambient illuminance.

The light control film may be arranged between the reflective polariserand the spatial light modulator. Advantageously the reflectivity of thedisplay is not reduced in privacy mode, and the luminance for off-axissnoopers is reduced, increasing security factor in privacy mode.

The spatial light modulator comprises an emissive spatial lightmodulator arranged to emit the spatially modulated light. Advantageouslythickness and cost may be reduced.

The display device further comprises a backlight arranged to outputlight, the spatial light modulator comprises a transmissive spatiallight modulator arranged to receive and spatially modulate the outputlight from the backlight, and the light control film may be arrangedbetween the backlight and the spatial light modulator. In comparison toemissive displays, off-axis luminance may be reduced and advantageouslysecurity factor increased. Backlights that use recirculated light may beused so that advantageously yield, uniformity and resilience to damageof the backlight may be increased.

The display device further comprises a backlight arranged to outputlight, the spatial light modulator comprises a transmissive spatiallight modulator arranged to receive and spatially modulate the outputlight from the backlight, and the light control film may be arranged infront of the spatial light modulator. Reduced scatter for off-axis lightmay be achieved and advantageously high angle luminance may be reducedso that security factor is increased.

The display device further comprises a backlight arranged to outputlight, the spatial light modulator comprises a transmissive spatiallight modulator arranged to receive and spatially modulate the outputlight from the backlight, and said display polariser may be an inputdisplay polariser arranged on the input side of the spatial lightmodulator. Frontal reflections to the primary head-on user may bereduced, advantageously increasing image contrast for head-on users inbright ambiently illuminated environments.

The light control film may be arranged between the backlight and theadditional polariser. Scatter and depolarisation in the polar controlretarder may be increased, advantageously achieving reduced off-axisluminance and increased security factor.

The at least one polar control retarder includes a switchable liquidcrystal retarder. The switchable liquid crystal retarder comprises alayer of liquid crystal material and at least one surface alignmentlayer disposed adjacent to the layer of liquid crystal material. Theswitchable liquid crystal retarder comprises two surface alignmentlayers disposed adjacent to the layer of liquid crystal material and onopposite sides thereof and arranged on respective liquid crystalencapsulation substrates. Advantageously a switchable privacy displaymay be provided with high image security for off-axis snoopers inprivacy mode and image visibility for off-axis users in public mode.

The light control film may be provided on one of the liquid crystalencapsulation substrates. Advantageously the thickness of the opticalstack may be reduced and the flatness of the light control film may beincreased to achieve increased uniformity.

The at least one polar control retarder further includes at least onepassive compensation retarder. Advantageously the size of the polarregion over which desirable image security is achieved in privacy modemay be increased.

The support substrate comprises at least one passive compensationretarder of the at least one passive compensation retarders.Advantageously the thickness of the optical stack may be reduced and theflatness of the light control film may be increased to achieve increaseduniformity.

Embodiments of the present disclosure may be used in a variety ofoptical systems. The embodiment may include or work with a variety ofprojectors, projection systems, optical components, displays,microdisplays, computer systems, processors, self-contained projectorsystems, visual and/or audio-visual systems and electrical and/oroptical devices. Aspects of the present disclosure may be used withpractically any apparatus related to optical and electrical devices,optical systems, presentation systems or any apparatus that may containany type of optical system. Accordingly, embodiments of the presentdisclosure may be employed in optical systems, devices used in visualand/or optical presentations, visual peripherals and so on and in anumber of computing environments.

Before proceeding to the disclosed embodiments in detail, it should beunderstood that the disclosure is not limited in its application orcreation to the details of the particular arrangements shown, becausethe disclosure is capable of other embodiments. Moreover, aspects of thedisclosure may be set forth in different combinations and arrangementsto define embodiments unique in their own right. Also, the terminologyused herein is for the purpose of description and not of limitation.

Directional backlights offer control over the illumination emanatingfrom substantially, the entire output surface controlled typicallythrough modulation of independent LED light sources arranged at theinput aperture side of an optical waveguide. Controlling the emittedlight directional distribution can achieve single person viewing for asecurity function, where the display can only be seen by a single viewerfrom a limited range of angles; high electrical efficiency, whereillumination is primarily provided over a small angular directionaldistribution; alternating left and right eye viewing for time sequentialstereoscopic and autostereoscopic display; and low cost.

These and other advantages and features of the present disclosure willbecome apparent to those of ordinary skill in the art upon reading thisdisclosure in its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example in the accompanyingFIGURES, in which like reference numbers indicate similar parts, and inwhich:

FIG. 1 is a schematic diagram illustrating in perspective side view aswitchable privacy display apparatus comprising a backlight comprisingcrossed brightness enhancement films, a light control film, atransmissive spatial light modulator with input and output displaypolarisers, a reflective polariser, a polar control retarder and anadditional polariser;

FIG. 2 is a schematic diagram illustrating in side view the privacydisplay of FIG. 1;

FIG. 3A is a schematic diagram illustrating in perspective side view aswitchable privacy display apparatus comprising a backlight comprising alight turning film, an additional polariser, a light control filmarranged on a passive retarder of a polar control retarder, a liquidcrystal retarder of a polar retarder and a transmissive spatial lightmodulator with input and output polarisers;

FIG. 3B is a schematic diagram illustrating in perspective side view aswitchable privacy display apparatus comprising a backlight comprising alight control film arranged on a light turning film, an additionalpolariser, a polar control retarder, and a transmissive spatial lightmodulator with input and output polarisers;

FIG. 4 is a schematic diagram illustrating in perspective side view apolar control retarder and light control film for a switchable privacydisplay apparatus wherein the support substrate of the light controlfilm is provided by a one of a pair of crossed A-plates;

FIG. 5 is a schematic diagram illustrating in perspective side view aswitchable privacy display apparatus comprising an emissive spatiallight modulator with an output display polariser, a light control film,a reflective polariser, a polar control retarder and an additionalpolariser;

FIG. 6 is a schematic diagram illustrating in perspective side view aswitchable privacy display apparatus comprising a spatial lightmodulator with an output display polariser, a light control film, apolar control retarder, an additional polariser, a light control filmwherein the support substrate of the light control film is provided withsensing electrodes of a touch screen;

FIG. 7A is a schematic diagram illustrating in perspective side view alight control film comprising an array of transmissive regions extendingbetween the input surface and the output surface, and absorptive regionsbetween the transmissive regions and extend at least partway between theinput surface and the output surface wherein the transmissive regionsare parallel;

FIG. 7B is a schematic diagram illustrating in perspective side view alight control film comprising an array of transmissive regions extendingbetween the input surface and the output surface, and absorptive regionsbetween the transmissive regions and extend at least partway between theinput surface and the output surface wherein the transmissive regionstapered;

FIG. 8 is a schematic graph illustrating variation with transmissionwith lateral angle for light control films;

FIG. 9A is a schematic diagram illustrating in top view the operation ofthe display apparatus of FIG. 1 in a privacy mode of operation;

FIG. 9B is a schematic diagram illustrating in top view the operation ofthe display, apparatus of FIG. 1 in a public mode of operation;

FIG. 10A is a schematic graph illustrating the variation with directionof luminance for a backlight comprising crossed brightness enhancementfilms;

FIG. 10B is a schematic graph illustrating the variation with directionof luminance for a backlight comprising a light control film that has atransmittance that is 5% or more in a range of polar angles in adirection in which the array of transmissive regions transmit that isgreater than 96° wide for all azimuthal angles;

FIG. 10C is a schematic graph illustrating the variation with directionof transmission of a polar control retarder of FIG. 1;

FIG. 10D is a schematic graph illustrating the variation with directionof reflection of a polar control retarder and reflective polariser ofFIG. 1;

FIG. 11 is a schematic graph illustrating the variation with directionof Fresnel reflection of a single surface in air;

FIG. 12 is a schematic graph illustrating variation of luminance withlateral angle for a display of the type illustrated in FIG. 1, thebacklight profile of FIG. 10A, and the transmission profiles of FIG. 10Band FIG. 10C for the display in privacy mode of operation;

FIG. 13 is a schematic graph illustrating the variation with directionof security, factor, S in privacy mode for a display of the typeillustrated in FIG. 1, comprising the backlight profile of FIG. 10A, thetransmission profiles of FIG. 10B and FIG. 10C, the reflection profileof FIG. 10D and for a display head-on luminance, of value Y_(max)measured in nits that is half of the illuminance of value I measured inlux;

FIG. 14 is a schematic graph illustrating variation of luminance withlateral angle for a display of the type illustrated in FIG. 1, thebacklight profile of FIG. 10A, and the transmission profile of FIG. 10Bfor the display in public mode of operation;

FIG. 15 is a schematic graph illustrating the variation with directionof security factor, S in public mode for a display of the typeillustrated in FIG. 1, comprising the backlight profile of FIG. 10A, thetransmission profile of FIG. 10B, the reflection profile of FIG. 11, andfor a display head-on luminance, of value Y_(max) measured in nits thatis half of the illuminance of value I measured in lux;

FIG. 16 is a schematic graph illustrating variation of luminance andtransmission with lateral angle for a prior art non-switchable privacydisplay;

FIG. 17 is a schematic graph illustrating the variation with directionof security factor, S in for a prior art non-switchable privacy display,comprising the backlight profile of FIG. 10A, and the transmissionprofile of a light control film comprising a lateral width of 70° atwhich the transmission is 5% of the head-on transmission, and thereflection profile of FIG. 11 for a display head-on luminance, of valueY_(max) measured in nits that is half of the illuminance of value Imeasured in lux;

FIG. 18 is a schematic diagram illustrating in perspective side view alight control film comprising an array of transmissive regions extendingbetween the input surface and the output surface, and absorptive regionsbetween the transmissive regions and extend at least partway between theinput surface and the output surface wherein the transmissive regionstapered, wherein the transmissive regions are tilted so that axesdefined in respect of each transmissive region between centres ofapertures of input and output ends of the transmissive regions aredirected inwardly towards an optical axis extending forwardly from thecentre of the spatial light modulator;

FIG. 19 is a schematic diagram illustrating in top view the operation ofa display apparatus of FIG. 1 comprising the light control film of FIG.18 in a privacy mode of operation;

FIG. 20 is a schematic diagram illustrating in top view a displayapparatus comprising a curved transmissive spatial light modulator,curved backlight, curved plural retarders and curved light control film;

FIG. 21 is a schematic diagram illustrating in top view a displayapparatus comprising a curved emissive spatial light modulator, curvedplural retarders and a curved light control film;

FIG. 22A is a schematic diagram illustrating in side view propagation ofoutput light from a spatial light modulator through the optical stack ofFIG. 1 in a privacy mode of operation;

FIG. 22B is a schematic diagram illustrating in top view propagation ofambient illumination light through the optical stack of FIG. 1 in aprivacy mode of operation;

FIG. 23A is a schematic diagram illustrating in side view propagation ofoutput light from a spatial light modulator through the optical stack ofFIG. 1 in a public mode of operation;

FIG. 23B is a schematic graph illustrating the variation of outputluminance with polar direction for the transmitted light rays in FIG.23A;

FIG. 23C is a schematic diagram illustrating in top view propagation ofambient illumination light through the optical stack of FIG. 1 in apublic mode of operation; and

FIG. 23D is a schematic graph illustrating the variation of reflectivitywith polar direction for the reflected light rays in FIG. 23C.

DETAILED DESCRIPTION

Terms related to optical retarders for the purposes of the presentdisclosure will now be described.

In a layer comprising a uniaxial birefringent material there is adirection governing the optical anisotropy whereas all directionsperpendicular to it (or at a given angle to it) have equivalentbirefringence.

The optical axis of an optical retarder refers to the direction ofpropagation of a light ray in the uniaxial birefringent material inwhich no birefringence is experienced. This is different from theoptical axis of an optical system which may for example be parallel to aline of symmetry or normal to a display surface along which a principalray propagates.

For light propagating in a direction orthogonal to the optical axis, theoptical axis is the slow axis when linearly polarized light with anelectric vector direction parallel to the slow axis travels at theslowest speed. The slow axis direction is the direction with the highestrefractive index at the design wavelength. Similarly the fast axisdirection is the direction with the lowest refractive index at thedesign wavelength.

For positive dielectric anisotropy uniaxial birefringent materials theslow axis direction is the extraordinary axis of the birefringentmaterial. For negative dielectric anisotropy uniaxial birefringentmaterials the fast axis direction is the extraordinary axis of thebirefringent material.

The terms half a wavelength and quarter a wavelength refer to theoperation of a retarder for a design wavelength λ₀ that may typically bebetween 500 nm and 570 nm. In the present illustrative embodimentsexemplary retardance values are provided for a wavelength of 550 nmunless otherwise specified.

The retarder provides a phase shift between two perpendicularpolarization components of the light wave incident thereon and ischaracterized by the amount of relative phase, Γ, that it imparts on thetwo polarization components; which is related to the birefringence Δnand the thickness d of the retarder by

Γ=2·π·Δn·d/λ ₀  eqn. 1

In eqn. 1, Δn is defined as the difference between the extraordinary andthe ordinary index of refraction, i.e.

Δn=n _(e) −n _(o)  eqn. 2

For a half-wave retarder, the relationship between d, Δn, and λ₀ ischosen so that the phase shift between polarization components is Γ=π.For a quarter-wave retarder, the relationship between d, Δn, and λ₀ ischosen so that the phase shift between polarization components is Γ=π/2.

The term half-wave retarder herein typically refers to light propagatingnormal to the retarder and normal to the spatial light modulator.

Some aspects of the propagation of light rays through a transparentretarder between a pair of polarisers will now be described.

The state of polarisation (SOP) of a light ray is described by therelative amplitude and phase shift between any two orthogonalpolarization components. Transparent retarders do not alter the relativeamplitudes of these orthogonal polarisation components but act only ontheir relative phase. Providing a net phase shift between the orthogonalpolarisation components alters the SOP whereas maintaining net relativephase preserves the SOP. In the current description, the SOP may betermed the polarisation state.

A linear SOP has a polarisation component with a non-zero amplitude andan orthogonal polarisation component which has zero amplitude.

A linear polariser transmits a unique linear SOP that has a linearpolarisation component parallel to the electric vector transmissiondirection of the linear polariser and attenuates light with a differentSOP.

Absorbing polarisers are polarisers that absorb one polarisationcomponent of incident light and transmit a second orthogonalpolarisation component. Examples of absorbing linear polarisers aredichroic polarisers.

Reflective polarisers are polarisers that reflect one polarisationcomponent of incident light and transmit a second orthogonalpolarisation component. Examples of reflective polarisers that arelinear polarisers are multilayer polymeric film stacks such as DBEF™ orAPF™ from 3M Corporation, or wire grid polarisers such as ProFlux™ fromMoxtek. Reflective linear polarisers may further comprise cholestericreflective materials and a quarter waveplate arranged in series.

A retarder arranged between a linear polariser and a parallel linearanalysing polariser that introduces no relative net phase shift providesfull transmission of the light other than residual absorption within thelinear polariser.

A retarder that provides a relative net phase shift between orthogonalpolarisation components changes the SOP and provides attenuation at theanalysing polariser.

In the present disclosure an ‘A-plate’ refers to an optical retarderutilizing a layer of birefringent material with its optical axisparallel to the plane of the layer.

A ‘positive A-plate’ refers to positively birefringent A-plates, i.e.A-plates with a positive Δn.

In the present disclosure a ‘C-plate’ refers to an optical retarderutilizing a layer of birefringent material with its optical axisperpendicular to the plane of the layer. A ‘positive C-plate’refers to apositively birefringent C-plate, i.e. a C-plate with a positive Δn. A‘negative C-plate’ refers to a negatively birefringent C-plate, i.e. aC-plate with a negative Δn.

‘O-plate’ refers to an optical retarder utilizing a layer ofbirefringent material with its optical axis having a component parallelto the plane of the layer and a component perpendicular to the plane ofthe layer. A ‘positive O-plate’ refers to positively birefringentO-plates, i.e. O-plates with a positive Δn.

Achromatic retarders may be provided wherein the material of theretarder is provided with a retardance Δn·d that varies with wavelengthλ as

Δn·d/λ=κ  eqn. 3

where κ is substantially a constant.

Examples of suitable materials include modified polycarbonates fromTeijin Films. Achromatic retarders may be provided in the presentembodiments to advantageously minimise colour changes between polarangular viewing directions which have low luminance reduction and polarangular viewing directions which have increased luminance reductions aswill be described below.

Various other terms used in the present disclosure related to retardersand to liquid crystals will now be described.

A liquid crystal cell has a retardance given by Δn·d where Δn is thebirefringence of the liquid crystal material in the liquid crystal celland d is the thickness of the liquid crystal cell, independent of thealignment of the liquid crystal material in the liquid crystal cell.

Homogeneous alignment refers to the alignment of liquid crystals inswitchable liquid crystal displays where molecules align substantiallyparallel to a substrate. Homogeneous alignment is sometimes referred toas planar alignment. Homogeneous alignment may typically be providedwith a small pre-tilt such as 2 degrees, so that the molecules at thesurfaces of the alignment layers of the liquid crystal cell are slightlyinclined as will be described below. Pretilt is arranged to minimisedegeneracies in switching of cells.

In the present disclosure, homeotropic alignment is the state in whichrod-like liquid crystalline molecules align substantiallyperpendicularly to the substrate. In discotic liquid crystalshomeotropic alignment is defined as the state in which an axis of thecolumn structure, which is formed by disc-like liquid crystallinemolecules, aligns perpendicularly to a surface. In homeotropicalignment, pretilt is the tilt angle of the molecules that are close tothe alignment layer and is typically close to 90 degrees and for examplemay be 88 degrees.

In a twisted liquid crystal layer a twisted configuration (also known asa helical structure or helix) of nematic liquid crystal molecules isprovided. The twist may be achieved by means of a non-parallel alignmentof alignment layers. Further, cholesteric dopants may be added to theliquid crystal material to break degeneracy of the twist direction(clockwise or anti-clockwise) and to further control the pitch of thetwist in the relaxed (typically undriven) state. A supertwisted liquidcrystal layer has a twist of greater than 180 degrees. A twisted nematiclayer used in spatial light modulators typically has a twist of 90degrees.

Liquid crystal molecules with positive dielectric anisotropy areswitched from a homogeneous alignment (such as an A-plate retarderorientation) to a homeotropic alignment (such as a C-plate or O-plateretarder orientation) by means of an applied electric field.

Liquid crystal molecules with negative dielectric anisotropy areswitched from a homeotropic alignment (such as a C-plate or O-plateretarder orientation) to a homogeneous alignment (such as an A-plateretarder orientation) by means of an applied electric field.

Rod-like molecules have a positive birefringence so that n_(e)>n_(o) asdescribed in eqn. 2. Discotic molecules have negative birefringence sothat n_(e)<n_(o).

Positive retarders such as A-plates, positive O-plates and positiveC-plates may typically be provided by stretched films or rod-like liquidcrystal molecules. Negative retarders such as negative C-plates may beprovided by stretched films or discotic like liquid crystal molecules.

Parallel liquid crystal cell alignment refers to the alignment directionof homogeneous alignment layers being parallel or more typicallyantiparallel. In the case of pre-tilted homeotropic alignment, thealignment layers may have components that are substantially parallel orantiparallel. Hybrid aligned liquid crystal cells may have onehomogeneous alignment layer and one homeotropic alignment layer. Twistedliquid crystal cells may be provided by alignment layers that do nothave parallel alignment, for example oriented at 90 degrees to eachother.

Transmissive spatial light modulators may further comprise retardersbetween the input display polariser and the output display polariser forexample as disclosed in U.S. Pat. No. 8,237,876, which is hereinincorporated by reference in its entirety. Such retarders (not shown)are in a different place to the passive retarders of the presentembodiments. Such retarders compensate for contrast degradations foroff-axis viewing locations, which is a different effect to the luminancereduction for off-axis viewing positions of the present embodiments.

Terms related to privacy display appearance will now be described.

A private mode of operation of a display is one in which an observersees a low contrast sensitivity such that an image is not clearlyvisible. Contrast sensitivity is a measure of the ability to discernbetween luminances of different levels in a static image. Inversecontrast sensitivity may be used as a measure of visual security, inthat a high visual security level (VSL) corresponds to low imagevisibility.

For a privacy display providing an image to an observer, visual securitymay be given as:

VSL=(Y+R)/(Y−K)  eqn. 4

where VSL is the visual security level, Y is the luminance of the whitestate of the display at a snooper viewing angle, K is the luminance ofthe black state of the display at the snooper viewing angle and R is theluminance of reflected light from the display.

Panel contrast ratio is given as:

C=Y/K  eqn. 5

For high contrast optical LCD modes, the white state transmissionremains substantially constant with viewing angle. In the contrastreducing liquid crystal modes of the present embodiments, white statetransmission typically reduces as black state transmission increasessuch that

Y+K˜P·L  eqn. 6

The visual security level may then be further given as:

$\begin{matrix}{{VSL} = \frac{( {C + {I \cdot {\rho/\pi} \cdot {( {C + 1} )/( {P \cdot L} )}}} )}{( {C - 1} )}} & {{eqn}.\mspace{14mu} 7}\end{matrix}$

where off-axis relative luminance, P is typically defined as thepercentage of head-on luminance, L at the snooper angle and the displaymay have image contrast ratio C and the surface reflectivity is ρ.

The off-axis relative luminance, P is sometimes referred to as theprivacy level. However, such privacy level P describes relativeluminance of a display at a given polar angle compared to head-onluminance, and is not a measure of privacy appearance.

The display may be illuminated by Lambertian ambient illuminance I. Thusin a perfectly dark environment, a high contrast display has VSL ofapproximately 1.0. As ambient illuminance increases, the perceived imagecontrast degrades, VSL increases and a private image is perceived.

For typical liquid crystal displays the panel contrast C is above 100:1for almost all viewing angles, allowing the visual security level to beapproximated to:

VSL=1+I·ρ/(π·P·L)  eqn. 8

The perceptual image security may be determined from the logarithmicresponse of the eye, such that the security factor, S is given by:

S=log₁₀(V)  eqn. 9

Desirable limits for S were determined in the following manner. In afirst step a privacy display device was provided. Measurements of thevariation of privacy level, P(θ) of the display, device with polarviewing angle and variation of reflectivity ρ(θ) of the display devicewith polar viewing angle were made using photopic measurement equipment.A light source such as a substantially uniform luminance light box wasarranged to provide illumination from an illuminated region that wasarranged to illuminate the privacy display device along an incidentdirection for reflection to a viewer positions at a polar angle ofgreater than 0° to the normal to the display device. The variation I(θ)of illuminance of a substantially Lambertian emitting lightbox withpolar viewing angle was determined by measuring the variation ofrecorded reflective luminance with polar viewing angle taking intoaccount the variation of reflectivity ρ(θ). The measurements of P(θ),r(θ) and I(θ) were used to determine the variation of Security FactorS(θ) with polar viewing angle along the zero elevation axis.

In a second step a series of high contrast images were provided on theprivacy display, including (i) small text images with maximum fontheight 3 mm, (ii) large text images with maximum font height 30 mm and(iii) moving images.

In a third step each observer (with eyesight correction for viewing at1000 mm where appropriate) viewed each of the images from a distance of1000 m, and adjusted their polar angle of viewing at zero elevationuntil image invisibility was achieved for one eye from a position nearon the display at or close to the centre-line of the display. The polarlocation of the observer's eye was recorded. From the relationship S(θ),the security factor at said polar location was determined. Themeasurement was repeated for the different images, for various displayluminance Y_(max), different lightbox illuminance I(q=0), for differentbackground lighting conditions and for different observers.

From the above measurements S<1.0 provides low or no visual security,1.0≤S<1.5 provides visual security that is dependent on the contrast,spatial frequency and temporal frequency of image content, 1.5≤S<1.8provides acceptable image invisibility (that is no image contrast isobservable) for most images and most observers and S≥1.8 provides fullimage invisibility, independent of image content for all observers.

In comparison to privacy displays, desirably wide-angle displays areeasily observed in standard ambient illuminance conditions. One measureof image visibility is given by the contrast sensitivity such as theMichelson contrast which is given by:

M=(I _(max) −I _(min))/(I _(max) +I _(min))  eqn. 10

and so:

M=((Y+R)−(K+R))/((Y+R)+(K+R))=(Y−K)/(Y+K+2·R)  eqn. 11

Thus the visual security level (VSL), is equivalent (but not identicalto) 1/M. In the present discussion, for a given off-axis relativeluminance, P the wide-angle image visibility, W is approximated as

W=1/VSL=1/(1+I·ρ/(π·P·L))  eqn. 12

In the present discussion the colour variation Δε of an output colour(u_(w)′+Δu′, v_(w)′+Δv′) from a desirable white point (u_(w)′, v_(w)′)may be determined by the CIELUV colour difference metric, assuming atypical display spectral illuminant and is given by:

Δε=(Δu′ ² +Δv′ ²)^(1/2)  eqn. 13

Catadioptric elements employ both refraction and reflection, which maybe total internal reflection or reflection from metallised surfaces.

The structure and operation of various directional display devices willnow be described. In this description, common elements have commonreference numerals. It is noted that the disclosure relating to anyelement applies to each device in which the same or correspondingelement is provided. Accordingly, for brevity such disclosure is notrepeated.

It may be desirable to provide high visual security levels for spatiallight modulators and/or backlights that provide high luminance inoff-axis viewing angles. The structure of a switchable privacy displaywill now be described.

FIG. 1 is a schematic diagram illustrating in perspective side view aswitchable privacy display apparatus 100 comprising a backlight 20comprising crossed brightness enhancement films 40A, 40B, a lightcontrol film 700, a transmissive spatial light modulator 48 with inputand output display polarisers 210, 218, a reflective polariser 302, apolar control retarder 300 and an additional polariser 318; and FIG. 2is a schematic diagram illustrating in side view the privacy display 100of FIG. 1.

Display device 100 comprises a spatial light modulator 48 arranged tooutput spatially modulated light. The display device 100 furthercomprises a backlight 20 arranged to output light and the spatial lightmodulator 48 comprises a transmissive spatial light modulator 48arranged to receive and spatially modulate the output light from thebacklight 20. Spatially-modulated light is provided by controllablepixels modulating the light from backlight 20. The transmissive spatiallight modulator 48 may comprise a liquid crystal display comprisingencapsulation substrates 212, 216, and liquid crystal layer 214 havingred, green and blue pixels 220, 222, 224. The spatial light modulator 48has an input display polariser 210 and an output display polariser 218on opposite sides thereof. The output display polariser 218 is arrangedto provide high extinction ratio for light from the pixels 220, 222, 224of the spatial light modulator 48. Typical polarisers 210, 218 may beabsorbing polarisers such as dichroic polarisers.

In the present embodiment the spatial light modulator 48 includes adisplay polariser 218 arranged on the output side of the spatial lightmodulator, the display polariser 218 being a linear polariser. Anadditional polariser 318 is arranged on the same side of the spatiallight modulator as the display polariser, the additional polariser 318being a linear polariser.

Backlight 20 will now be further described. The backlight apparatus 20comprises a rear reflector 3; and an illumination apparatus comprisingwaveguide 1 and light sources 15. Light rays 412 from the source 15 areinput through input side 2 and guide within the surfaces 6, 8 of thewaveguide 1. Light is output by means of extraction features 12 and isincident onto rear reflector 3 which may reflect light either byscattering or specular reflection back through the waveguide 1 andtowards crossed brightness enhancement films 40A, 40B that are arrangedto receive light exiting from the first surface 6 of waveguide 1. In thepresent embodiments, ‘crossed’ refers to an angle of substantially 90°between the optical axes of the two retarders in the plane of theretarders.

Brightness enhancement films 40A, 40B each comprise a prismatic layerwith prismatic surfaces 42A, 42B arranged between the optical waveguide1 and the spatial light modulator 48 to receive output light from theoptical waveguide 1. Light rays 412 from the waveguide direct the outputlight through the spatial light modulator 48.

The prismatic surfaces 42A, 42B are elongate; and the orientation of theelongate prismatic surfaces of the turning film and further turning filmare crossed. Light that is in directions near to the optical axis 199are reflected back towards the reflector 3, whereas light rays 410 thatare closer to grazing the surface 6 is output in the normal direction.

Optical stack 5 may comprise diffusers, light turning films and otherknown optical backlight structures. Asymmetric diffusers, that maycomprise asymmetric surface relief features for example, may be providedin the optical stack 5 with increased diffusion in the elevationdirection in comparison to the lateral direction. Advantageously imageuniformity may be increased.

Optionally reflective polariser 208 may be provided between the inputdisplay polariser 210 and backlight 20 to provide recirculated light andincrease display efficiency. Advantageously efficiency may be increased.

The light recirculating components 3, 40A, 40B, 208 of backlight 20achieve a mixing of output light from the waveguide. Such recirculationis tolerant to manufacturing defects and backlights 20 mayadvantageously be provided with larger size, lower cost and higherluminance uniformity than the collimated backlights that will beillustrated with reference to FIG. 3A, below. However, such backlightsprovide increased luminance at higher polar angles that may degradesecurity factor in privacy mode of operation as will be described below.

It would be desirable to provide high uniformity backlights with lowmanufacturing cost while achieving high security factor in privacy mode,and achieving desirable luminance in the public mode of operation.

A light control film 700 is arranged between the backlight 20 and thespatial light modulator 48. The light control film 700 comprises aninput surface 706, an output surface 708 facing the input surface 706,an array of light transmissive regions 704 extending between the inputsurface 706 and the output surface 708, and absorptive regions 702between the transmissive regions and extending between the input surfaceand the output surface.

Light control film 700 is arranged between the reflective polariser 208of the backlight 20 and the display input polariser 210. Light controlfilm 700 may further comprise a support substrate 710. Advantageouslythe flatness of the light control film may be increased to achieveincreased uniformity. The structure and operation of the light controlfilm will be further described hereinbelow.

It would be desirable to provide a switchable privacy display. Lightcontrol film 700 is arranged in series with the spatial light modulator48, the additional polariser 318 and a polar control retarder 300.

Polar control retarder 300 comprises: (i) a switchable liquid crystalretarder 301 comprising a layer 314 of liquid crystal material arrangedbetween transparent encapsulation substrates 312, 316 and arrangedbetween the display polariser 218 and the additional polariser 318; and(ii) at least one passive compensation retarder 330.

Polar control retarder 300 is arranged between the additional polariser318 and the display polariser 218. The general principles of operationof polar control retarders 300 arranged between polariser 218, 302, 318will be described hereinbelow with respect to FIG. 22A to FIG. 23D.

The polar control retarder 300 includes a switchable liquid crystalretarder 301. The switchable liquid crystal retarder 301 comprises alayer 314 of liquid crystal material surface alignment layers 409, 411disposed adjacent to the layer 314 of liquid crystal material. Theswitchable liquid crystal retarder 301 comprises two surface alignmentlayers 409, 411 disposed adjacent to the layer of liquid crystalmaterial 314 and on opposite sides thereof and arranged on respectiveliquid crystal encapsulation substrates 312, 316. Further electrodes413, 415 are arranged to provide a drive voltage across the liquidcrystal layer 314. In a privacy mode of operation, a first ac voltage isapplied by driver 350 and in a public mode of operation a second acvoltage that may be zero or a different voltage to the privacy mode isapplied by driver 350 across electrodes 413, 415.

The display device 100 further comprises a reflective polariser 302arranged between the output display polariser 218 and at least one firstpolar control retarder 300, the reflective polariser 302 being a linearpolariser. Reflective polariser 302 is different in function andoperation to the reflective polariser 208 described above. Reflectivepolariser 302 achieves increased frontal reflection and increasedsecurity factor while reflective polariser 208 achieves increasedrecirculation efficiency and uniformity in a backlight.

In alternative embodiments (not shown), a further polar control retardermay be arranged between the input polariser 210 and a further additionalpolariser arranged between the backlight and the input polariser 210. Infurther alternative embodiments (not shown), a further additionalpolariser may be arranged between the reflective polariser 302 andoutput polariser 218. A further polar control retarder may be arrangedbetween the output polariser 218 and the further additional polariser.Further additional polarisers and further additional polar controlretarders may advantageously achieve increased luminance to off-axisusers in a public mode of operation, and may achieve narrower switch-onangles for desirable image security factor in privacy mode of operation.

An alternative display structure will now be illustrated.

FIG. 3A is a schematic diagram illustrating in perspective side view aswitchable privacy display apparatus comprising a backlight 20comprising a light turning film 50, an additional polariser 318, a lightcontrol film 700 arranged on a passive C-plate retarder 330 of a polarcontrol retarder 300, a liquid crystal retarder 301 of the polarretarder 300 and a transmissive spatial light modulator 48 with inputand output polarisers 210, 218. The display device 100 comprises abacklight 20 arranged to output light, the spatial light modulator 48comprises a transmissive spatial light modulator 48 arranged to receiveand spatially modulate the output light from the backlight 20, and saiddisplay polariser is an input display polariser 210 arranged on theinput side of the spatial light modulator 48.

FIG. 3A illustrates an alternative to FIG. 1 wherein the polar controlretarder 300 is arranged between the additional polariser 318 and theinput polariser 210 of the spatial light modulator 48. Reflectivepolariser 302 is omitted. The frontal reflection from the display 100 isreduced, advantageously achieving increased contrast for the display inhigh illuminance. Further the front-of display thickness is reduced,achieving increased image resolution in embodiments providing scatteringoutput surfaces of polariser 218.

Light control film 700 is arranged between the additional polariser 318and display polariser 210, FIG. 3A illustrates another alternative toFIG. 1 wherein the support substrate 710 of the light control film 700is omitted and the light control film 700 is provided on the passiveretarder 330. Advantageously thickness and cost may be reduced, anduniformity, may be maintained.

In alternative embodiments (not shown) light control film 700 may beattached to 312, 316. Advantageously thickness may be reduced, and theflatness of the light control film 700 may be achieved, increasinguniformity of output.

FIG. 3A further shows one alternative backlight 20 in comparison to thebacklight 20 of FIG. 1 and FIG. 2. In comparison to FIG. 1, the rearreflector 3 is a specular reflector and the waveguide has surfacestructure arranged to provide output light near to grazing output anglesat the output surface 6 of the waveguide 1. As will be described in FIG.15, advantageously such a backlight can achieve a narrower opticaloutput solid angle than the optical output of the backlight 20 ofFIG. 1. Advantageously a privacy display with increased security factorand with a narrower switch-on angle for privacy may be provided.

Features of the embodiment of FIG. 3A not discussed in further detailmay be assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

FIG. 3B is a schematic diagram illustrating in perspective side view aswitchable privacy display apparatus 100 comprising a backlight 20comprising a light control film 700 arranged on light turning film 50,an additional polariser 318, a polar control retarder 300, and atransmissive spatial light modulator 48 with input and output polarisers210, 218.

The light control film 700 is arranged between the backlight 20 and theadditional polariser 318. Residual retardance, scatter and stray lightin the light control film does not reduce the contrast of the polarcontrol retarder for off-axis viewing locations. Advantageously, thesecurity factor for off-axis snoopers may be increased in comparison tothe arrangement of FIG. 3A.

FIG. 3B illustrates another alternative substrate for the light controlfilm provided by light turning film 50. Advantageously thickness andcost may be reduced and uniformity may be increased.

Features of the embodiment of FIG. 3B not discussed in further detailmay be assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

FIG. 4 is a schematic diagram illustrating in perspective side view apolar control retarder 300 and light control film 700 for a switchableprivacy display apparatus wherein the support substrate of the lightcontrol film 700 is provided by a one of a pair of crossed A-plates330A, 330B. In comparison to the C-plate retarder of FIG. 1, FIG. 2 andFIG. 3A, A-plates may provide increased region of high security factorin liquid crystal modes with homogeneous alignment of alignment layers409, 411.

The light control film 700 may be provided on the passive retarder 330A.Advantageously thickness and cost may be reduced, and uniformity may bemaintained. The polar control retarder 300 of FIG. 4 may be providedbetween the additional polariser 318 and the input polariser 210 orbetween the additional polariser 318 and an output polariser 218 of thespatial light modulator 48.

Features of the embodiment of FIG. 4 not discussed in further detail maybe assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

It may be desirable to provide a switchable privacy display using anemissive spatial light modulator 48.

FIG. 5 is a schematic diagram illustrating in perspective side view aswitchable privacy display apparatus 100 comprising an emissive spatiallight modulator 48 with an output display polarise 218, a light controlfilm 700, a reflective polarise 302, a polar control retarder 300 and anadditional polariser 318.

Emissive spatial light modulator further comprises quarter waveplate 205and display polariser 218 that are arranged to reduce visibility ofreflections from the pixel plane 214 of the spatial light modulator 48.

The spatial light modulator 48 comprises an emissive spatial lightmodulator arranged to emit the spatially modulated light. Spatial lightmodulator 48 may be an OLED display or a micro-LED display with an arrayof self-emitting pixels 220, 222, 224 in comparison to the transmissivepixels of FIG. 1.

Light control film 700 is arranged between the reflective polariser 302and the spatial light modulator 48. Light control film 700 is arrangedbetween the reflective polariser 302 and the display polariser 218.

In alternative embodiments the emissive spatial light modulator 48 ofFIG. 5 may be provided by a transmissive spatial light modulator 48 andbacklight 20 as described elsewhere herein. The backlight 20 may providelower luminance at high polar angles in comparison to the emissiondirections of emissive spatial light modulator 48. Advantageouslyluminance at high angles may be reduced to achieve increased securityfactor for off-axis snoopers in privacy mode of operation.

Features of the embodiment of FIG. 5 not discussed in further detail maybe assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

FIG. 6 is a schematic diagram illustrating in perspective side view aswitchable privacy display apparatus 100 comprising a spatial lightmodulator 48 with an output display polariser 218, a polar controlretarder 300, an additional polariser 318, a light control film 700wherein the support substrate of the light control film is provided withelectrodes of a touch screen.

The display device 100 further comprises a backlight 20 arranged tooutput light, the spatial light modulator comprises a transmissivespatial light modulator 48 arranged to receive and spatially modulatethe output light from the backlight 20, and the light control film isarranged in front of the spatial light modulator 48.

Electrodes 500 and drivers 452 are arranged on the support substrate 710and further electrodes 502 and drivers 454 are provided on substrate510, with dielectric 501 provided between substrates 710, 510. A finger25 in close proximity may modulate projected field lines 570, 572 thatmay be detected to provide a touch input. The light control film 700 isseparated from the pixel plane 214 and Moiré may advantageously bereduced. The number of substrates may be reduced, advantageouslyreducing thickness and cost.

Features of the embodiment of FIG. 6 not discussed in further detail maybe assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

The structure and operation of the light control film 700 will now bedescribed.

FIG. 7A is a schematic diagram illustrating in perspective side view alight control film 700 that comprises an input surface 706, an outputsurface 708 facing the input surface 706, an array of light transmissiveregions 704 extending between the input surface 706 and the outputsurface 708, and absorptive regions 702 between the transmissive regionsand extending partway between the input surface 706 and the outputsurface 708.

The array of transmissive regions 704 is a one-dimensional array ofelongate transmissive regions 704, that are elongate in the y-axisdirection.

The absorptive regions 702 between the transmissive regions 704 extendbetween the input surface 706 and the output surface 708. The lightcontrol film 700 is provided on a support substrate 710. The substrate710 may be arranged on the input side 706 or the output side 708 of thelight control film.

The light transmissive regions 704 are parallel sided and have surfacenormals in the plane of the light control film 700 such that thedirection of maximum light transmission, for example ray 490 is normalto the plane of the light control film 700. The transmissive regions 704have axes 709 defined in respect of each transmissive region 704 betweencentres 705, 707 of apertures 716, 718 of input and output ends 706, 708of the transmissive regions 704 are normal to the plane (x-y plane) ofthe spatial light modulator 48 at all positions across the light controlfilm 700.

In the arrangement of FIG. 7A, the light control film 700 has atransmittance that has a profile with polar angle in a direction inwhich the array of transmissive regions repeats (x-axis) that is centredon the normal to the plane (x-y plane) of the spatial light modulator 48at all positions across the light control film.

In the embodiment of FIG. 7A, light absorbing region 704 do not extendbetween the input side and output side 706, 708 of the light controlfilm. Rather, a layer 709 of transmissive material may be providedacross the light control film. Such a layer may provide support for thelight transmissive regions 704 during fabrication.

Features of the embodiment of FIG. 7A not discussed in further detailmay be assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

An alternative structure of light control film will now be described.

FIG. 7B is a schematic diagram illustrating in perspective side view alight control film 700 comprising an array of transmissive regions 702extending between the input surface 706 and the output surface 708, andabsorptive regions 702 between the transmissive regions 704 andextending between the input surface 706 and the output surface 708wherein the transmissive regions 704 tapered such that the width S_(IN)of the aperture 716 on the input side is less than the width of theaperture 718 S_(OUT) on the output side 708. In comparison to the lightcontrol film 700 of FIG. 7A, such an arrangement may advantageouslyprovide increased luminance uniformity in on-axis directions as will bedescribed further below.

Features of the embodiment of FIG. 7B not discussed in further detailmay be assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

The optical transmission of the structures of FIGS. 7A-B will now bedescribed.

FIG. 8 is a schematic graph illustrating variation with transmissionwith lateral angle in the direction in which the light transmissiveregions 704 repeat for various light control films 700.

Under illumination, the structure of FIG. 7A would be expected toprovide a triangular profile with incident angle in the lateral angle,that is the transmission is maximum in the direction 199 that is normalto the plane of the film 700. At non-zero lateral angles some light raysthat pass through the input aperture 716 of the light transmittingregions 704 is incident onto the absorbing regions 702 and so luminancefalls. In an idealised non-scatting arrangement, such a profile providesa substantially triangular profile. In reality some light may betransmitted by the light absorbing regions 704. Further, scatterincluding that from diffraction spreads light to provide a modifiedtriangular profile, with some rounding of profile near the axis and nearthe angle at which otherwise all light would be absorbed. It isconvenient to describe the angular range of polar angles 460 provided bythe light control film 700 using the angle at which the transmission ofthe film falls to 5% of the input illumination. This is different to thetransmission of the film compared to the head-on transmission due tolosses at the light-absorbing regions 702 for on-axis light rays.

Known non-switchable privacy displays that are provided by the useradding louver films to the front of conventional spatial lightmodulators will now be described.

Profile 456 is for a known prior art light control film for use in onetype of non-switchable privacy display. Such a profile provides a rangeof polar angles 460 of 66°, that is the angle for 5% transmission. At45°, the luminance is reduced to provide some degree of image securityfor use in privacy. As will be described further below, such a film foruse in a non-switchable privacy display provides undesirably low imagevisibility to an off-axis viewer in public operation. Further such afilm provides a head-on transmission of about 70%, such a loss providesundesirable loss of luminance and/or increased power consumption.

Profile 454 is for a different known prior art light control film foruse in another type of non-switchable privacy display that provides apassive luminance reduction and passive reflectivity increase off-axis.Such a profile provides a range of polar angles 460 of 72°. At 45°, theluminance is reduced and reflectivity increased to provide increasedimage security for privacy operation. However, such a film providesundesirable off-axis luminance and off-axis reflectivity in publicoperation.

Profiles 450, 452 are desirable profiles for embodiments of the presentdisclosure. Such profiles are not desirable profiles for use innon-switchable privacy displays as they provide too high luminance atoff-axis angles to achieve desirable security factors for off-axissnoopers.

Profile 450 may be provided by light transmitting regions that areparallel sided, that is S_(OUT)=S_(IN) and as illustrated in FIG. 7A.Advantageously profile 450 has higher head-on transmission and tools foruse in replication of the light control film 700 may be conveniently,fabricated, for example by means of photoresist exposure in collimatedlight through a mask.

Profile 452 may be provided by light transmitting regions that aretapered, that is S_(OUT)>S_(IN) and as illustrated in FIG. 7B.Advantageously profile 452 provides increased uniformity in regionsclose to the user viewing direction, increasing display uniformity.

Light control film 700 of the present disclosure may have atransmittance that is 5% or more in a range of polar angles in adirection in which the array of transmissive regions repeats that is atleast 80° wide and may have a transmittance that is 5% or more in arange of polar angles in a direction in which the array of transmissiveregions repeats that is at least 90° wide.

Further the light control film 700 may have a transmittance that is 5%or more in a range of polar angles in a direction in which the array oftransmissive regions repeats that is at most 130° wide.

The absorptive regions 702 of the light control film 700 may have athickness, t, wherein t is given by the expression:

t=(S _(IN) −S _(IN) ²/10p+S _(OUT))/(2*tan(a sin(ξ/n))  eqn. 14

where S_(IN) is a width of an aperture 716 of the input end 706 of theabsorptive regions, S_(OUT) is a width of an aperture 718 of the outputend of the absorptive regions 702, p is a pitch of the transmissiveregions 704 in the direction in which the array of transmissive regionsrepeats, and n is the refractive index of the transmissive regions;wherein ξ may be 0.643 or more or ξ may be 0.707 or more. Further ξ maybe 0.906 or less.

In an exemplary embodiment a light control film 700 such thatillustrated in FIG. 7A provides a profile 450 with a range of polarangles 460 of 98° (for which ξ=0.755), with a transmission of greaterthan 15% at 45°. Such a profile advantageously achieves hightransmission efficiency head-on.

In a further exemplary embodiment a light control film 700 such thatillustrated in FIG. 7B provides a profile 452 with a range of polarangles 460 of 106° (for which ξ=0.799), with a transmission of greaterthan 20% at 45°. Such a profile advantageously achieves an extendedregion of uniform transmission near on-axis directions. Desirably theuniformity of the display as seen by the head-on user may be increasedin comparison to profile 450.

The thickness 706 of the layer 700, and width of louvres 702, 704 isselected to provide an absorption at 45 degrees in at least oneazimuthal orientation that is between 2% and 30% of the absorption inthe direction normal to the light control film, preferably between 4%and 20% of the absorption in the direction normal to the light controlfilm and most preferably between 6% and 10% of the absorption in thedirection normal to the light control film 700.

The operation of the displays of FIGS. 1-6 in privacy and public modesof operation will now be described in overview.

FIG. 9A is a schematic diagram illustrating in top view the operation ofthe display apparatus of FIG. 1 in a privacy mode of operation; and FIG.9B is a schematic diagram illustrating in top view the operation of thedisplay apparatus of FIG. 1 in a public mode of operation. Features ofthe embodiments of FIGS. 9A-B not discussed in further detail may beassumed to correspond to the features with equivalent reference numeralsas discussed above, including any potential variations in the features.

Light cone 480 is output from backlight 20 (or emissive spatial lightmodulator 48) and directed through light control film 700 withtransmission light cone 482 that is narrower than the cone 480.

In FIG. 9A, the polar control retarder 300 is arranged to reduceoff-axis luminance, with cone size 484. Head-on observer 45 sees lightrays 445 from across the display with high luminance and high imagevisibility. Off-axis snooper 47 sees light rays 447 from across thedisplay with reduced luminance and increased image security. Someregions of the display that are closer to the snooper 47 may undesirablyhave higher luminance.

In a public mode of operation as illustrated in FIG. 9B the cone width486 is substantially increased and both users can see light from acrossthe display with high image visibility within the transmission cone 482of the light control film 700 and backlight 20.

Illustrative embodiments of a switchable privacy display will now bedescribed. In the present illustrative embodiments, the field of view600 as seen by user 45 in a plane at 500 mm and a lateral angle of 0degrees from a 14″ landscape display will be marked and the field ofview 602 as seen by user 47 in a plane at 700 mm from the display and ata lateral angle of 45 degrees are illustrated.

FIG. 10A is a schematic graph illustrating the variation with directionof luminance for a backlight 20 comprising crossed brightnessenhancement films such as that illustrated in FIG. 1. FIG. 10B is aschematic graph illustrating the variation with direction of luminancefor a backlight comprising a light control film 700 that has atransmittance that is 5% or more in a range of polar angles 460 in adirection in which the array of transmissive regions 704 repeat that is100° (for which ξ=0.766).

An illustrative embodiment of polar control retarder 300 will now begiven.

FIG. 10C is a schematic graph illustrating the variation with directionof transmission of a polar control retarder of FIG. 1 for theillustrative embodiment of the arrangement is given in TABLE 1; and FIG.10D is a schematic graph illustrating the variation with direction ofreflectivity of a polar control retarder of FIG. 1 for the illustrativeembodiment of the arrangement is given in TABLE 1 wherein the display100 further comprises a reflective polariser 302.

TABLE 1 Alignment LC layer 314 Passive retarder 330 Passive retarder 330type retardance type retardance Homogeneous 1000 nm Homeotropic NegativeC-plate −800 nm

FIG. 11 is a schematic graph illustrating the variation with directionof Fresnel reflection of a single surface in air. Such reflections areincluded in calculations of Security Factor, S and Image Visibility, Wfor both public and privacy modes of operation.

Further non-limiting alternatives of polarisation control retarder 300will now be described.

In one alternative, the switchable liquid crystal retarder 301 maycomprise two surface alignment layers disposed adjacent to the layer 413of liquid crystal material 414 and on opposite sides thereof and eacharranged to provide homeotropic alignment in the adjacent liquid crystalmaterial. The layer 413 of liquid crystal material 414 of the switchableliquid crystal retarder 301 may comprise a liquid crystal material witha negative dielectric anisotropy. The layer 413 of liquid crystalmaterial 414 may have a retardance for light of a wavelength of 550 nmin a range from 500 nm to 1000 nm, preferably in a range from 600 nm to900 nm and most preferably, in a range from 700 nm to 850 nm.

Where two surface alignment layers providing homeotropic alignment areprovided, the at least one passive compensation retarder 330 maycomprise a retarder having its optical axis perpendicular to the planeof the retarder, the at least one passive retarder having a retardancefor light of a wavelength of 550 nm in a range from −300 nm to −900 nm,preferably in a range from −450 nm to −800 nm and most preferably in arange from −500 nm to −725 nm.

Alternatively, where two surface alignment layers providing homeotropicalignment are provided, the at least one passive compensation retarder330A, 330B may comprise a pair of retarders which have optical axes inthe plane of the retarders that are crossed, each retarder of the pairof retarders having a retardance for light of a wavelength of 550 nm ina range from 300 nm to 800 nm, preferably in a range from 500 nm to 700nm and most preferably in a range from 550 nm to 675 nm. Advantageously,in this case increased field of view in wide angle mode of operation maybe provided. Further, zero voltage operation in wide angle mode ofoperation may be provided, reducing power consumption.

In another alternative, the switchable liquid crystal retarder 301 maycomprise two surface alignment layers disposed adjacent to the layer 413of liquid crystal material 414 and on opposite sides thereof and eacharranged to provide homogeneous alignment in the adjacent liquid crystalmaterial. Advantageously in comparison to homeotropic alignment onopposite sides of the liquid crystal, increased resilience to thevisibility of flow of liquid crystal material during applied pressuremay be achieved.

The layer 413 of liquid crystal material 414 of the switchable liquidcrystal retarder 301 may comprise a liquid crystal material with apositive dielectric anisotropy. The layer 413 of liquid crystal material414 may have a retardance for light of a wavelength of 550 nm in a rangefrom 500 nm to 900 nm, preferably in a range from 600 nm to 850 nm andmost preferably in a range from 700 nm to 800 nm.

Where two surface alignment layers providing homogeneous alignment areprovided, the at least one passive compensation retarder 330 maycomprise a retarder having its optical axis perpendicular to the planeof the retarder, the at least one passive retarder having a retardancefor light of a wavelength of 550 nm in a range from −300 nm to −700 nm,preferably in a range from −350 nm to −600 nm and most preferably in arange from −400 nm to −500 nm.

Alternatively, where the two surface alignment layers providinghomogeneous alignment are provided, the at least one passivecompensation retarder 330A, 330B may comprise a pair of retarders whichhave optical axes in the plane of the retarders that are crossed, eachretarder of the pair of retarders having a retardance for light of awavelength of 550 nm in a range from 300 nm to 800 nm, preferably in arange from 350 nm to 650 nm and most preferably in a range from 450 nmto 550 nm. Advantageously, in this case increased resilience to thevisibility of flow of liquid crystal material during applied pressuremay be achieved.

In another alternative, the switchable liquid crystal retarder 301 maycomprise two surface alignment layers disposed adjacent to the layer 413of liquid crystal material 414 and on opposite sides thereof, one of thesurface alignment layers being arranged to provide homeotropic alignmentin the adjacent liquid crystal material and the other of the surfacealignment layers being arranged to provide homogeneous alignment in theadjacent liquid crystal material.

When the surface alignment layer arranged to provide homogeneousalignment is between the layer 413 of liquid crystal material 414 andthe compensation retarder 330, the layer 413 of liquid crystal material414 may have a retardance for light of a wavelength of 550 nm in a rangefrom 700 nm to 2000 nm, preferably in a range from 1000 nm to 1500 nmand most preferably in a range from 1200 nm to 1500 nm.

When the surface alignment layer arranged to provide homogeneousalignment is between the layer 413 of liquid crystal material 414 andthe compensation retarder 330, the at least one passive compensationretarder 330 may comprise a retarder having its optical axisperpendicular to the plane of the retarder, the at least one passiveretarder having a retardance for light of a wavelength of 550 nm in arange from −400 nm to −1800 nm, preferably in a range from −700 nm to−1500 nm and most preferably in a range from −900 nm to −1300 nm.

When the surface alignment layer arranged to provide homogeneousalignment is between the layer 413 of liquid crystal material 414 andthe compensation retarder 330A, 330B, the at least one passivecompensation retarder 330A, 330B may comprise a pair of retarders whichhave optical axes in the plane of the retarders that are crossed, eachretarder of the pair of retarders having a retardance for light of awavelength of 550 nm in a range from 400 nm to 1800 nm, preferably in arange from 700 nm to 1500 nm and most preferably in a range from 900 nmto 1300 nm.

When the surface alignment layer arranged to provide homeotropicalignment is between the layer 413 of liquid crystal material 414 andthe compensation retarder 330, the layer 413 of liquid crystal material414 may have a retardance for light of a wavelength of 550 nm in a rangefrom 500 nm to 1800 nm, preferably in a range from 700 nm to 1500 nm andmost preferably in a range from 900 nm to 1350 nm.

When the surface alignment layer arranged to provide homeotropicalignment is between the layer 413 of liquid crystal material 414 andthe compensation retarder 330, the at least one passive compensationretarder 330 may comprise a retarder having its optical axisperpendicular to the plane of the retarder, the at least one passiveretarder having a retardance for light of a wavelength of 550 nm in arange from −300 nm to −1600 nm, preferably in a range from −500 nm to−1300 nm and most preferably in a range from −700 nm to −1150 nm.

When the surface alignment layer arranged to provide homeotropicalignment is between the layer 413 of liquid crystal material 414 andthe compensation retarder 330A, 330B, the at least one passivecompensation retarder 330A, 330B may comprise a pair of retarders whichhave optical axes in the plane of the retarders that are crossed, eachretarder of the pair of retarders having a retardance for light of awavelength of 550 nm in a range from 400 nm to 1600 nm, preferably in arange from 600 nm to 1400 nm and most preferably in a range from 800 nmto 1300 nm. Advantageously, in this case increased resilience to thevisibility of flow of liquid crystal material during applied pressuremay be achieved.

Each alignment layer may have a pretilt having a pretilt direction witha component in the plane of the liquid crystal layer that is parallel oranti-parallel or orthogonal to the electric vector transmissiondirection of the display polariser. Advantageously a display may beprovided with narrow viewing angle in a lateral direction and a wideviewing freedom for display rotation about a horizontal axis. Such adisplay may be comfortable to view for a head-on display user anddifficult to view for an off-axis display user.

The at least one passive retarder may comprise at least two passiveretarders with at least two different orientations of optical axes whichmay have optical axes in the plane of the retarders that are crossed.Field of view for liquid crystal retarders with homogeneous alignment isincreased while providing resilience to the visibility of flow of liquidcrystal material during applied pressure.

The pair of passive retarders may have optical axes that extend at 45°and at 135°, respectively, with respect to an electric vectortransmission direction that is parallel to the electric vectortransmission of the display polariser. The passive retarders may beprovided using stretched films to advantageously achieve low cost andhigh uniformity.

The switchable liquid crystal retarder 301 may be provided between thepair of passive retarders. Advantageously the thickness and complexityof the plural retarders may be reduced.

A transparent electrode and a liquid crystal alignment layer may beformed on a side of each of the pair of passive retarders adjacent theswitchable liquid crystal retarder 301; and may further comprise firstand second substrates between which the switchable liquid crystalretarder 301 is provided, the first and second substrates eachcomprising one of the pair of passive retarders, wherein each of thepair of passive retarders has a retardance for light of a wavelength of550 nm in a range from 150 nm to 800 nm, preferably in a range from 200nm to 700 nm and most preferably in a range from 250 nm to 600 nm.

In one alternative, the at least one passive compensation retarder 330may comprise a retarder having an optical axis perpendicular to theplane of the retarder. Advantageously the thickness and complexity ofthe passive retarder stack may be reduced.

The at least one passive compensation retarder 330A, 330B may comprisetwo passive retarders having an optical axis perpendicular to the planeof the passive retarders, and the switchable liquid crystal retarder 301is provided between the two passive retarders. Advantageously thethickness and complexity of the plural retarders may be reduced. Highhead-on efficiency may be achieved in both wide and privacy modes, awide field of view for wide angle mode and snoopers may be unable toperceive image data from a wide range of off-axis viewing locations.

A transparent electrode and a liquid crystal alignment layer may beformed on a side of each of the two passive retarders adjacent theswitchable liquid crystal retarder 301. First and second substratesbetween which the switchable liquid crystal retarder 301 may beprovided, the first and second substrates each comprising one of the twopassive retarders. The two passive retarders may have a total retardancefor light of a wavelength of 550 nm in a range from −300 nm to −700 nm,preferably in a range from −350 nm to −600 nm and most preferably in arange from −400 nm to −500 nm.

In another alternative, the at least one passive compensation retarder330 may comprise a retarder having an optical axis with a componentperpendicular to the plane of the retarder and with a component in theplane of the retarder. Advantageously fields of view in wide angle modemay be increased and snoopers may be unable to perceive image data froma wide range of off-axis viewing locations.

The component in the plane of the passive retarder may extend at 0°,with respect to an electric vector transmission direction that isparallel or perpendicular to the electric vector transmission of thedisplay polariser. The at least one passive retarder may furthercomprise a passive retarder having an optical axis perpendicular to theplane of the passive retarder or a pair of passive retarders which haveoptical axes in the plane of the passive retarders that are crossed.

The retardance of the at least one passive compensation retarder 330 maybe equal and opposite to the retardance of the switchable liquid crystalretarder 301.

The switchable liquid crystal retarder 301 may comprise first and secondpretilts; and the at least one passive compensation retarder 330 maycomprise a compensation retarder 330 with first and second pretilts, thefirst pretilt of the compensation retarder 330 being the same as thefirst pretilt of the liquid crystal retarder and the second pretilt ofthe compensation retarder 330 being the same as the second pretilt ofthe liquid crystal retarder.

The switchable liquid crystal retarder 301 may further compriseelectrodes arranged to apply a voltage for controlling the layer 413 ofliquid crystal material 414. The electrodes may be on opposite sides ofthe layer 413 of liquid crystal material 414. The display may beswitched by control of the liquid crystal layer, advantageouslyachieving a switchable privacy display, or other display with reducedoff-axis stray light. The display may further comprise a control systemarranged to control the voltage applied across the electrodes of the atleast one switchable liquid crystal retarder 301.

The output of display 100 comprising the components of FIGS. 10A-D willnow be described.

FIG. 12 is a schematic graph illustrating variation of normalisedluminance with lateral angle for a display of the type illustrated inFIG. 1, the backlight profile of FIG. 10A, and the transmission profilesof FIG. 10B and FIG. 10C. Profile 430 is the lateral variation ofbacklight luminance of FIG. 10A. Profile 432 is the lateral variation oflight control film 700 transmittance of FIG. 10B. Profile 434 is thevariation of the polar control retarder of FIG. 10C in privacy mode ofoperation. Profile 436 is the resultant profile of display 100 outputluminance in privacy mode of operation.

FIG. 13 is a schematic graph illustrating the variation with directionof security factor, S in privacy mode for a display of the typeillustrated in FIG. 1, comprising the backlight profile of FIG. 10A, thetransmission profiles of FIG. 10B and FIG. 10C, the reflection profileof FIG. 10D and for a display head-on luminance, of value Y_(max)measured in nits that is half of the illuminance of value I measured inlux. The head-on observer 45 has a field of view 600 that provides highimage visibility (S<0.1) across the whole of the display. Advantageouslya bright and easily read image is seen. The off-axis user 47 has a fieldof view 610 for which advantageously all image data is private (S≥1.0)and some of the display is invisible for all image data (S≥1.8).

FIG. 14 is a schematic graph illustrating variation of normalisedluminance with lateral angle for a display of the type illustrated inFIG. 1, the backlight profile of FIG. 10A, and the transmission profilesof FIG. 10B and FIG. 10C. Profile 430 is the lateral variation ofbacklight luminance of FIG. 10A. Profile 432 is the lateral variation oflight control film 700 transmittance of FIG. 10B. Profile 434 is thevariation of a polar control retarder in public mode of operation.Profile 436 is the resultant profile of display 100 output luminance inpublic mode of operation. The display 100 achieves an angular width 438of 90° for which the luminance is 5% or greater of the head-onluminance.

FIG. 15 is a schematic graph illustrating the variation with directionof security factor, S in public mode for a display of the typeillustrated in FIG. 1, comprising the backlight profile of FIG. 10A, thetransmission profile of FIG. 10B, the reflection profile of FIG. 11, andfor a display head-on luminance, of value Y_(max) measured in nits thatis half of the illuminance of value I measured in lux. The head-onobserver 45 has a field of view 600 that provides high image visibility(S<0.1) across the whole of the display. Advantageously a bright andeasily read image is seen. The off-axis user 47 has a field of view 610for which advantageously no image data is invisible (S<1.0).Advantageously data can be shared by multiple users.

The operation of a prior art non-switchable privacy display will now bedescribed.

FIG. 16 is a schematic graph illustrating variation of luminance andtransmission with lateral angle for an exemplary prior artnon-switchable privacy display. Said prior art display may be providedby a backlight with the same luminance profile as FIG. 10A and lateralluminance profile 430. Attached to the output of said display by theuser is a prior art light control film with a transmittance that is 5%or more in a range of polar angles in a direction in which the array oftransmissive regions repeats that is 70° wide. The output luminanceprofile of said prior art display is illustrated by profile 437.

FIG. 17 is a schematic graph illustrating the variation with directionof security factor, S in for a prior art non-switchable privacy display,comprising the backlight profile of FIG. 10A, the transmission profileof a light control film comprising a lateral width of 70° at which thetransmission is 5% of the head-on transmission, and the front surfacereflection profile of FIG. 11 for a display head-on luminance, of valueY_(max) measured in nits that is half of the illuminance of value Imeasured in lux.

Such a display has a single mode of operation, such that the displaycharacteristics are fixed. Considering the off-axis snooper field ofview 610, when it is desirable to hide off-axis information such adisplay achieves a privacy level S>1.0, however some types of image datais visible across the display as S<1.5. Advantageously by way ofcomparison the present embodiments in FIG. 13 achieve increased imagesecurity and image invisibility for off-axis snoopers.

Considering the off-axis user field of view 610, when it is desirable toshow off-axis information, however some types of image data content isinvisible across the display as S>1.0. Advantageously by way ofcomparison the present embodiments in FIG. 15 achieve increased imagevisibility for off-axis users.

In other words, light control film 700 of the present embodiments with atransmittance that is 5% or more in a range of polar angles in adirection in which the array of transmissive regions repeats that is 80°wide (ξ>0.643) can achieve increased image visibility in public mode andincreased image security in privacy mode in comparison to the prior artdisplay.

It would be desirable to increase the luminance and security leveluniformity of a privacy display.

FIG. 18 is a schematic diagram illustrating in perspective side view alight control film 700 for a display device 100 of the presentembodiments; and FIG. 19 is a schematic diagram illustrating in top viewthe operation of a display apparatus of FIG. 1 comprising the lightcontrol film of FIG. 18 in a privacy mode of operation.

The light control film 700 comprises an array of transmissive regions704 extending between the input surface 706 and the output surface 708,and absorptive regions 702 between the transmissive regions. Absorptiveregions 704 extend between the input surface 706 and the output surface708 wherein the transmissive regions 704 are tapered. The transmissiveregions 702 are tilted so that axes 709 defined in respect of eachtransmissive region 704 between centres 705, 707 of apertures 716, 718of input and output ends of the transmissive regions 704 are directedinwardly towards an optical axis 440 extending forwardly from the centreof the spatial light modulator 48.

In other words the light control film 700 has a transmittance that hasprofiles with polar angle in a direction in which the array oftransmissive regions repeats that have centre lines 709 directedinwardly towards an optical axis 440 extending forwardly from the centreof the spatial light modulator 448. Said centre lines 709 of saidprofiles are directed towards a common point 442.

The transmissive regions 704 are tilted so that axes 709 defined inrespect of each transmissive region 704 between centres 705, 707 ofapertures 716, 718 of input and output ends 706, 708 of the transmissiveregions 704 are directed inwardly towards an optical axis 709 extendingforwardly from the centre of the spatial light modulator 48 and saidaxes are directed towards a common point 442.

In comparison to the arrangement of FIG. 9A, light cones 482 are thustilted towards the head-on user for image points across the display inthe direction in which the transmissive regions 704 repeat. The centreof the light cones 482 are directed towards the user 45 and thus theuniformity of the image seen is increased, as roll-offs in the profiledue to the transmission of the profile 432 of FIG. 14 for example areless visible or not visible. Advantageously display uniformity isincreased. Further the off-axis snooper sees increased uniformity at lowlight levels for points across the display, increasing the uniformity ofthe security factor S, in comparison to that illustrated by field ofview 610 in FIG. 13. Advantageously increased image security isachieved.

Further such a pupillation of light towards the point 442 is achieved bythe tapered light transmission regions, achieving increased uniformityof illumination. Such tapers may be pupillated without surfaces thatoverhang, providing a tool that may be suitable for replication of thelight transmissive regions 704.

Features of the embodiment of FIG. 18 and FIG. 19 not discussed infurther detail may be assumed to correspond to the features withequivalent reference numerals as discussed above, including anypotential variations in the features.

Embodiments comprising curved light control film 700 to improveluminance uniformity and security factor uniformity will now bedescribed.

FIG. 20 is a schematic diagram illustrating in top view a displayapparatus comprising a curved transmissive spatial light modulator 48,curved backlight 20, curved polar control retarder 300 and curved lightcontrol film 700; and FIG. 21 is a schematic diagram illustrating in topview a display apparatus comprising a curved emissive spatial lightmodulator 48, curved polar control retarder and a curved light controlfilm 700.

Light from image points across the display 100 are directed towards acommon point 442 by means of the curvature of the respective elements.The common point 442 may be at the nominal viewing distance for observer45 as illustrated in FIG. 20 or as an alternative may be at a differentdistance. Preferably the distance is the same or greater than thenominal viewing distance as illustrated in FIG. 21.

In comparison to the arrangement of FIG. 9A, light cones 480, 482, 484are tilted towards the head-on user 45 across the display. Variations inthe luminance profiles 430, 432, 434 of FIG. 14 for example (andcorresponding front reflectivity profile when reflective polarise 302 isprovided) are less visible or not visible across the display.Advantageously, display uniformity is increased. Further the off-axissnooper sees increased uniformity of luminance at low light levels (anduniformity of reflectivity when reflective polariser 302 is provided)for points across the display device 100, increasing the uniformity ofthe security factor S, in comparison to that illustrated by field ofview 610 in FIG. 13. Advantageously, increased image security isachieved.

Features of the embodiment of FIGS. 20-21 not discussed in furtherdetail may be assumed to correspond to the features with equivalentreference numerals as discussed above, including any potentialvariations in the features.

The principles of operation of the switchable polar control retarders ofFIG. 1 will now be described.

FIG. 22A is a schematic diagram illustrating in side view propagation ofoutput light from a spatial light modulator through the optical stack ofFIG. 1 in a privacy mode of operation.

When the layer 314 of liquid crystal material 414 is driven to operatein the privacy mode, the retarders 300 provide no overall transformationof polarisation component 360 to output light rays 400 passingtherethrough along an axis perpendicular to the plane of the switchableretarder, but provides an overall transformation of polarisationcomponent 361 to light rays 402 passing therethrough for some polarangles which are at an acute angle to the perpendicular to the plane ofthe retarders.

Polarisation component 360 from the output polariser 218 is transmittedby reflective polariser 302 and incident on retarders 300. On-axis lighthas a polarisation component 362 that is unmodified from component 360while off-axis light has a polarisation component 364 that istransformed by the retarders 300. At a minimum, the polarisationcomponent 361 is transformed to a linear polarisation component 364 andabsorbed by additional polariser 318. More generally, the polarisationcomponent 361 is transformed to an elliptical polarisation component,that is partially absorbed by additional polariser 318.

The polar distribution of light transmission illustrated in FIG. 10Amodifies the polar distribution of luminance output of the underlyingspatial light modulator 48. In the case that the spatial light modulator48 comprises a directional backlight 20 then off-axis luminance may befurther be reduced as described above.

Features of the embodiment of FIG. 22A not discussed in further detailmay be assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

Advantageously, a privacy display is provided that has low luminance toan off-axis snooper while maintaining high luminance for an on-axisobserver.

The operation of the reflective polariser 302 for light from ambientlight source 604 will now be described for the display operating inprivacy mode.

FIG. 22B is a schematic diagram illustrating in top view propagation ofambient illumination light through the optical stack of FIG. 1 in aprivacy mode of operation.

Ambient light source 604 illuminates the display device 100 withunpolarised light. Additional polariser 318 transmits light ray 410normal to the display device 100 with a first polarisation component 372that is a linear polarisation component parallel to the electric vectortransmission direction 319 of the additional polariser 318.

In both states of operation, the polarisation component 372 remainsunmodified by the retarders 300 and so transmitted polarisationcomponent 382 is parallel to the transmission axis of the reflectivepolariser 302 and the output polariser 218, so ambient light is directedthrough the spatial light modulator 48 and lost.

By comparison, for ray 412, off-axis light is directed through theretarders 300 such that polarisation component 374 incident on thereflective polarise 302 may be reflected. Such polarisation component isre-converted into component 376 after passing through retarders 300 andis transmitted through the additional polariser 318.

Thus when the layer 314 of liquid crystal material is in the secondstate of said two states, the reflective polariser 302 provides noreflected light for ambient light rays 410 passing through theadditional polariser 318 and then the retarders 300 along an axisperpendicular to the plane of the retarders 300, but provides reflectedlight rays 412 for ambient light passing through the additionalpolariser 318 and then the retarders 300 at some polar angles which areat an acute angle to the perpendicular to the plane of the retarders300; wherein the reflected light 412 passes back through the retarders300 and is then transmitted by the additional polariser 318.

The retarders 300 thus provide no overall transformation of polarisationcomponent 380 to ambient light rays 410 passing through the additionalpolariser 318 and then the retarder 300 along an axis perpendicular tothe plane of the switchable retarder, but provides an overalltransformation of polarisation component 372 to ambient light rays 412passing through the absorptive polariser 318 and then the retarders 300at some polar angles which are at an acute angle to the perpendicular tothe plane of the retarders 300.

The polar distribution of light reflection illustrated in FIG. 10D thusillustrates that high reflectivity can be provided at typical snooperlocations by means of the privacy state of the retarders 300. Thus, inthe privacy mode of operation, the reflectivity for off-axis viewingpositions is increased as illustrated in FIG. 10D, and the luminance foroff-axis light from the spatial light modulator is reduced asillustrated in FIG. 10A.

In the public mode of operation, the control system 710, 752, 350 isarranged to switch the switchable liquid crystal retarder 301 into asecond retarder state in which a phase shift is introduced topolarisation components of light passing therethrough along an axisinclined to a normal to the plane of the switchable liquid crystalretarder 301.

By way of comparison, solid angular extent 402D may be substantially thesame as solid angular extent 402B in a public mode of operation. Suchcontrol of output solid angular extents 402C, 402D may be achieved bysynchronous control of the sets 15, 17 of light sources and the at leastone switchable liquid crystal retarder 300.

Advantageously a privacy mode may be achieved with low image visibilityfor off-axis viewing and a large solid angular extent may be providedwith high efficiency for a public mode of operation, for sharing displayimagery between multiple users and increasing image spatial uniformity.

Additional polariser 318 is arranged on the same output side of thespatial light modulator 48 as the display output polariser 218 which maybe an absorbing dichroic polariser. The display polariser 218 and theadditional polariser 318 have electric vector transmission directions219, 319 that are parallel. As will be described below, such parallelalignment provides high transmission for central viewing locations.

A transmissive spatial light modulator 48 arranged to receive the outputlight from the backlight; an input polariser 210 arranged on the inputside of the spatial light modulator between the backlight 20 and thespatial light modulator 48; an output polariser 218 arranged on theoutput side of the spatial light modulator 48; an additional polariser318 arranged on the output side of the output polariser 218; and aswitchable liquid crystal retarder 300 comprising a layer 314 of liquidcrystal material arranged between the at least one additional polariser318 and the output polariser 318 in this case in which the additionalpolariser 318 is arranged on the output side of the output polariser218; and a control system 710 arranged to synchronously, control thelight sources 15, 17 and the at least one switchable liquid crystalretarder 300.

Control system 710 further comprises control of voltage controller 752that is arranged to provide control of voltage driver 350, in order toachieve control of switchable liquid crystal retarder 301.

Features of the embodiment of FIG. 22B not discussed in further detailmay be assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

Advantageously, a privacy display is provided that has high reflectivityto an off-axis snooper while maintaining low reflectivity for an on-axisobserver. As described above, such increased reflectivity providesenhanced privacy performance for the display in an ambiently illuminatedenvironment.

Operation in the public mode will now be described.

FIG. 23A is a schematic diagram illustrating in side view propagation ofoutput light from a spatial light modulator through the optical stack ofFIG. 1 in a public mode of operation; and FIG. 23B is a schematic graphillustrating the variation of output luminance with polar direction forthe transmitted light rays in FIG. 23A.

Features of the embodiment of FIG. 23A and FIG. 23B not discussed infurther detail may be assumed to correspond to the features withequivalent reference numerals as discussed above, including anypotential variations in the features.

When the liquid crystal retarder 301 is in a first state of said twostates, the retarders 300 provide no overall transformation ofpolarisation component 360, 361 to output light passing therethroughperpendicular to the plane of the switchable retarder 301 or at an acuteangle to the perpendicular to the plane of the switchable retarder 301.That is polarisation component 362 is substantially the same aspolarisation component 360 and polarisation component 364 issubstantially the same as polarisation component 361. Thus the angulartransmission profile of FIG. 23B is substantially uniformly transmittingacross a wide polar region. Advantageously a display may be switched toa wide field of view.

FIG. 23C is a schematic diagram illustrating in top view propagation ofambient illumination light through the optical stack of FIG. 1 in apublic mode of operation; and FIG. 23D is a schematic graph illustratingthe variation of reflectivity with polar direction for the reflectedlight rays in FIG. 23C.

Thus when the liquid crystal retarder 301 is in the first state of saidtwo states, the retarders 300 provide no overall transformation ofpolarisation component 372 to ambient light rays 412 passing through theadditional polariser 318 and then the retarders 300, that isperpendicular to the plane of the retarders 300 or at an acute angle tothe perpendicular to the plane of the retarders 300.

In operation in the public mode, input light ray 412 has polarisationstate 372 after transmission through the additional polariser 318. Forboth head-on and off-axis directions no polarisation transformationoccurs and thus the reflectivity for light rays 402 from the reflectivepolariser 302 is low. Light ray 412 is transmitted by reflectivepolariser 302 and lost in the display polarisers 218, 210 or thebacklight of FIG. 1.

Features of the embodiment of FIG. 23C and FIG. 23D not discussed infurther detail may be assumed to correspond to the features withequivalent reference numerals as discussed above, including anypotential variations in the features.

Advantageously in a public mode of operation, high luminance and lowreflectivity is provided across a wide field of view. Such a display canbe conveniently viewed with high contrast by multiple observers.

Other types of switchable privacy display will now be described.

A display device 100 that may be switched between privacy and publicmodes of operation comprises an imaging waveguide and an array of lightsources as described in U.S. Pat. No. 9,519,153, which is incorporatedby reference herein in its entirety. The imaging waveguide images anarray of light sources to optical windows that may be controlled toprovide high luminance on-axis and low luminance off-axis in a privacymode, and high luminance with a large solid angle cone for publicoperation.

As may be used herein, the terms “substantially” and “approximately”provide an industry-accepted tolerance for its corresponding term and/orrelativity between items. Such an industry-accepted tolerance rangesfrom zero percent to ten percent and corresponds to, but is not limitedto, component values, angles, et cetera. Such relativity between itemsranges between approximately zero percent to ten percent.

While various embodiments in accordance with the principles disclosedherein have been described above, it should be understood that they havebeen presented by way of example only, and not limitation. Thus, thebreadth and scope of this disclosure should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with any claims and their equivalents issuing from thisdisclosure. Furthermore, the above advantages and features are providedin described embodiments, but shall not limit the application of suchissued claims to processes and structures accomplishing any or all ofthe above advantages.

Additionally, the section headings herein are provided for consistencywith the suggestions under 37 CFR 1.77 or otherwise to provideorganizational cues. These headings shall not limit or characterize theembodiment(s) set out in any claims that may issue from this disclosure.Specifically and by way of example, although the headings refer to a“Technical Field,” the claims should not be limited by the languagechosen under this heading to describe the so-called field. Further, adescription of a technology in the “Background” is not to be construedas an admission that certain technology is prior art to anyembodiment(s) in this disclosure. Neither is the “Summary” to beconsidered as a characterization of the embodiment(s) set forth inissued claims. Furthermore, any reference in this disclosure to“invention” in the singular should not be used to argue that there isonly a single point of novelty in this disclosure. Multiple embodimentsmay be set forth according to the limitations of the multiple claimsissuing from this disclosure, and such claims accordingly define theembodiment(s), and their equivalents, that are protected thereby. In allinstances, the scope of such claims shall be considered on their ownmerits in light of this disclosure, but should not be constrained by theheadings set forth herein.

1. A display device comprising: a spatial light modulator arranged tooutput spatially modulated light, the spatial light modulator includinga display polariser arranged on a side of the spatial light modulator,the display polariser being a linear polariser; an additional polariserarranged on the same side of the spatial light modulator as the displaypolariser, the additional polariser being a linear polariser; at leastone switchable liquid crystal retarder arranged between the additionalpolariser and the display polariser, and a light control film arrangedin series with the spatial light modulator, the additional polariser andthe at least one switchable liquid crystal retarder, wherein the lightcontrol film comprises an input surface, an output surface facing theinput surface, an array of transmissive regions extending between theinput surface and the output surface, and absorptive regions between thetransmissive regions and extending at least partway between the inputsurface and the output surface.
 2. A display device according to claim1, wherein the light control film has a transmittance that is 5% or morein a range of polar angles in a direction in which the array oftransmissive regions repeats that is at least 80° wide.
 3. A displaydevice according to claim 2, wherein the light control film has atransmittance that is 5% or more in a range of polar angles in adirection in which the array of transmissive regions repeats that is atleast 90° wide.
 4. A display device according to claim 2, wherein thelight control film has a transmittance that is 5% or more in a range ofpolar angles in a direction in which the array of transmissive regionsrepeats that is at most 130° wide.
 5. A display device according toclaim 1, wherein the absorptive regions of the light control film have athickness, t, wherein t is given by the expressiont=(S _(IN) −S _(IN) ²/10p+S _(OUT))/(2*tan(a sin(ξ/n)) where S_(IN) is awidth of an aperture of the input end of the absorptive regions, S_(OUT)is a width of an aperture of the output end of the absorptive regions, pis a pitch of the transmissive regions in the direction in which thearray of transmissive regions repeats, and n is the refractive index ofthe transmissive regions; wherein ξ is 0.643 or more.
 6. A displaydevice according to claim 5, wherein ξ is 0.707 or more.
 7. A displaydevice according to claim 5, wherein ξ is 0.906 or less.
 8. A displaydevice according to claim 1, wherein the light control film has atransmittance that has profiles with polar angle in a direction in whichthe array of transmissive regions repeats that have centre linesdirected inwardly towards an optical axis extending forwardly from thecentre of the spatial light modulator.
 9. A display device according toclaim 8, wherein said centre lines of said profiles are directed towardsa common point.
 10. A display device according to claim 1, wherein thelight control film has a transmittance that has a profile with polarangle in a direction in which the array of transmissive regions repeatsthat is centred on the normal to the plane of the spatial lightmodulator at all positions across the light control film.
 11. A displaydevice according to claim 1, wherein the transmissive regions are tiltedso that axes defined in respect of each transmissive region betweencentres of apertures of input and output ends of the transmissiveregions are directed inwardly towards an optical axis extendingforwardly from the centre of the spatial light modulator.
 12. A displaydevice according to claim 11, wherein said axes are directed towards acommon point.
 13. A display device according to claim 1, wherein thetransmissive regions have axes defined in respect of each transmissiveregion between centres of apertures of input and output ends of thetransmissive regions are normal to the plane of the spatial lightmodulator at all positions across the light control film.
 14. A displaydevice according to claim 1, wherein the array of transmissive regionsis a one-dimensional array of elongate transmissive regions.
 15. Adisplay device according to claim 1, wherein the absorptive regionsbetween the transmissive regions extend between the input surface andthe output surface.
 16. A display device according to claim 1, whereinthe light control film is provided on a support substrate.
 17. A displaydevice according to claim 1, wherein said display polariser is an outputdisplay polariser arranged on the output side of the spatial lightmodulator.
 18. A display device according to claim 17, wherein thedisplay device further comprises a reflective polariser arranged betweenthe output display polariser and at least one first switchable liquidcrystal retarder, the reflective polariser being a linear polariser. 19.A display device according to claim 18, wherein the light control filmis arranged between the reflective polariser and the spatial lightmodulator.
 20. A display device according to claim 17, wherein thespatial light modulator comprises an emissive spatial light modulatorarranged to emit the spatially modulated light.
 21. A display deviceaccording to claim 17, wherein the display device further comprises abacklight arranged to output light, the spatial light modulatorcomprises a transmissive spatial light modulator arranged to receive andspatially modulate the output light from the backlight, and the lightcontrol film is arranged between the backlight and the spatial lightmodulator.
 22. A display device according to claim 17, wherein thedisplay device further comprises a backlight arranged to output light,the spatial light modulator comprises a transmissive spatial lightmodulator arranged to receive and spatially modulate the output lightfrom the backlight, and the light control film is arranged in front ofthe spatial light modulator.
 23. A display device according to claim 1,wherein the display device further comprises a backlight arranged tooutput light, the spatial light modulator comprises a transmissivespatial light modulator arranged to receive and spatially modulate theoutput light from the backlight, and said display polariser is an inputdisplay polariser arranged on the input side of the spatial lightmodulator.
 24. A display device according to claim 23, wherein the lightcontrol film is arranged between the backlight and the additionalpolariser.
 25. (canceled)
 26. A display device according to claim 1,wherein the switchable liquid crystal retarder comprises a layer ofliquid crystal material and at least one surface alignment layerdisposed adjacent to the layer of liquid crystal material.
 27. A displaydevice according to claim 26, wherein the switchable liquid crystalretarder comprises two surface alignment layers disposed adjacent to thelayer of liquid crystal material and on opposite sides thereof andarranged on respective liquid crystal encapsulation substrates.
 28. Adisplay device according to claim 27, wherein the light control film isprovided on one of the liquid crystal encapsulation substrates.
 29. Adisplay device according to claim 1, wherein the at least one switchableliquid crystal retarder includes at least one passive compensationretarder.
 30. A display device according to claim 29, wherein thesupport substrate comprises at least one passive compensation retarderof the at least one passive compensation retarders.
 31. A display deviceaccording to claim 1, wherein at least one of the light control film andthe switchable liquid crystal retarder is curved.
 32. A display deviceaccording to claim 21, wherein at least one of the light control film,the switchable liquid crystal retarder and the backlight is curved.